Lots of useful things, according to panelists at the World Science Festival.

Don't call neutrinos elusive. Francis Halzen, who makes it his job to detect them, said that the particles' unusual properties, like their minuscule masses and tendency not to interact with matter, tend to make "elusive" one of the most common adjectives used to describe them. But in reality, they're all around us: each cubic centimeter of the Universe has hundreds of them, left over from the Big Bang. Every second, trillions of them flow through our bodies. In fact, there's probably as much matter in the form of neutrinos as there is visible matter. If we could just get a bit better at detecting them, they could tell us a lot about the Universe.

The World Science Festival hosted a discussion led by three people who have focused their careers on detecting neutrinos. Halzen helps run the new Ice Cube detector at the South Pole, while MIT's Janet Conrad works on the MiniBooNE detector, which picks up neutrinos made by the Fermilab accelerator chain. Conrad was joined by her fellow MITer, Joe Formaggio, who works on the Sudbury Neutrino Observatory, buried in a mine in Canada.

The panel was rounded out by theoretician Lawrence Krauss, and moderated by noted science writer John Rennie.

So why have neutrinos picked up the reputation of being hard to get a handle on, even though they're just about everywhere? Part of it is historical; the particles were predicted decades before anyone figured out how to confirm they actually existed. They are very hard to detect because their primary means of interaction with other matter is through the weak force, which only governs nuclear reactions like fusion and radioactive decay. Of course, that "only" is a rather big deal, since Halzen noted without the reactions that involve neutrinos, we wouldn't produce neutrons, and the Universe wouldn't have much in the way of chemistry.

The association with nuclear reactions and tendency not to interact with matter make them the best way of observing what goes on in the interior of the Sun. Krauss said that it takes photons about a million years to escape from the core of the Sun because there's so much matter to interact with. When we first tried to use neutrinos to probe the Sun, however, there didn't appear to be enough of them being emitted. On its own, the apparent shortage wasn't that surprising—as Krauss said, the average solar neutrino would go through about 10,000 light years of lead without interacting with anything, and we're just lucky that the Sun produces a lot of neutrinos.

But the deficit kept showing up in additional experiments. Eventually, we found out why: neutrinos, through a process called flavor oscillations, sometimes change their identity. The missing neutrinos were there—they just didn't look like we expected them to. And that was a bit of a surprise. "The interference between flavors only works cleanly if there are mass differences," Formaggio said, "which means some of them must have mass." As Krauss described it, "if they didn't have mass but did change flavors, then there would be no way to distinguish them."

A neutrino with mass is the clearest indication that the Standard Model has to be either wrong or incomplete. In fact, a physicist in the audience pointed out that, had we known neutrinos had mass early enough, we might never have accepted the Standard Model in the first place.

Halzen called the identification of neutrinos' mass the biggest discovery in particle physics—while noting that it didn't come from a particle accelerator. In fact, it came from an instrument made to look at something else entirely, the possibility that protons decay. (That study came up empty.)

But we still don't know what that mass is, as the flavor oscillations only tell us what the relative masses of the three types of neutrinos are. Formaggio compared that to having a car dealership tell you, "'I can get you $500 off the car'—you kind of still want to know what the price is." What we do know, in part from information in the radiation left over from the Big Bang, is that it's tiny. While the lightest particle with a known mass, the electron, is a half Mega-electron-volt, the upper limit on the mass of the neutrinos is less than one electron-volt.

Formaggio said that there are a couple of ways we might be able to get a measurement of the neutrino's mass: analysis of their effects on galaxy clusters or an identification of mass that's carried away from particle decays without registering in a detector. "Hopefully, they'll get the same number," he said, at which point both Krauss and Conrad argued that life is more fun when the numbers don't agree.

But mass isn't the only challenge facing neutrino physicists. There have been some hints that neutrinos might be the only fundamental particle that is its own antiparticle (making them Majorana fermions, for the technically inclined). This, Conrad said, would explain the fact that all the neutrinos we see have their spin oriented in one direction, while the antineutrinos have the opposite spin. She suggested that a rare form of radioactive decay that should produce two neutrinos might help us sort this out.

If neutrinos were their own antiparticles, these two should annihilate at an appreciable rate, leaving the decay neutrino-less. But, just days after the session, SLAC's EXO-200 experiment announced that it hadn't seen neutrino-less decays, making this prospect much less likely. (Also this week, Fermilab announced that new data had eliminated the possibility that antineutrino masses were different from those of regular neutrinos.)

What's next for neutrino work? Some cosmological data hints there might be a fourth type of neutrino, one that may not interact via the weak force. The Fermi announcement mentioned above indicated that its Minos detector would now focus on testing whether we can see an indication of these so-called "sterile" neutrinos on Earth.

Another big goal is the detection of the neutrinos that remain from the Big Bang, which were formed as the first atoms in the Universe condensed. They should, much like the cosmic microwave background, preserve information about the state of the early Universe. But the "neutrino background" is very low energy, which makes the neutrinos themselves much harder to detect. Formaggio said that, if large atoms are kept cold enough, it might be possible to detect a neutrino interacting with an entire nucleus, rather than just a proton or neutron. So far, however, no hardware capable of this has been built.

Meanwhile, Conrad has been inspired by people who ask her about the practical impact of her work. "What have neutrinos done for you lately?" she asked, rhetorically. "I get that all the time." Her answer is that they could do useful things because of their association with nuclear decays. So she's working on getting neutrino detectors down in size from something like MiniBooNE (which is 40 feet tall) to something that's portable enough that it could be moved to a site like Fukushima in order to tell us something about what's going on there.

29 Reader Comments

That was very interesting.The whole "What have neutrinos done for you lately" concept from Conrads side troubles me a little. Has science (or perhaps grants) come to the point that science for the sake of knowledge is no longer enough? Must it lead to tangible benefits and inventions?

Edit to clarify: I know LHC and other experiments are pure basic research sake. It is just sad to see scientists feel the need to justify work that "only" yields knowledge.

Thanks for getting me to contemplate the idea of 10000 light years of lead.

Seems like neutrinos have quite a few tricks up their sleeves.

I'm sure someone can solve this:

What would the radius of a 10,000 lightyear long cylinder of lead need to be to form a black hole?

Two ways of doing it. What volume of lead do you need to exceed neutron star mass limit (~ 3 Msun, this is the realistic way to do it) or what spherical volume of lead is inside its own event horizon (more fun, let's do it first).

The lead would collapse to a sphere under its own gravity pretty much no matter how thin you make the cylinder, so we can use the Schwarzschild radius. I get that the Schwarzschild radius is Rs = Sqrt[3c^2 / 8*G*pi*d] for density d.

Equivalently, Rs = (1.2*10^13 / Sqrt[d]) meters, for d in kg/m^3

The density of lead is 11340 kg/m^3. So, pure lead forms a black hole (we're assuming here no gravitational collapse past lead's crystal state...very bad assumption but hey, this is back of envelope...see next method) at a radius of about 1.2*10^11 m.

This corresponds to a volume of 7*10^33 m^3. A cylinder of height 10000 light years = 9.5*10^19 m requires a base radius of 7.4*10^13 m to have this volume. So, by this method, the cylinder collapses to a black hole at radius 7.4*10^13 m (keeping in mind this is a BAD METHOD that assumes magical support for the lead crystal).

By the other method, we need our cylinder to have mass 3 Msun = 6 * 10^30 kg.

Mcyl = Vcyl * d, Vcyl = Mcyl / d, Rcyl = Sqrt[Mcyl / pi*h*d]

Rcyl = 1.3 km (this is much closer to the actual limit)

*Found my error. I had missed a factor of G in the first method. I'm surprised at how large the cylinder can be, actually. The sun weighs A LOT.

That was very interesting.The whole "What have neutrinos done for you lately" concept from Conrads side troubles me a little. Has science (or perhaps grants) come to the point that science for the sake of knowledge is no longer enough? Must it lead to tangible benefits and inventions?

Though I don't share that view myself, I can easily understand some people wondering about that, considering the current economical situation. (I live in the Euro-zone.) Heck, if the world worked like Civ where you can freely up- or downgrade your research budget and shift your priorities at any time, I'd vote for a science downgrade myself, right now, since some of the current social and economical problems may bite us long before we see the fruits of these science projects. Unfortunately, in real life, lost funding hardly ever returns in better times, and projects and facilities can't be put into inexpensive limbo as easily as in Sid-Meier-land.

So, all things considered, I'm in favor of continuing research, but I can see how its value and continued funding may currently not be self-evident to everyone - including those handing out the grants.

That was very interesting.The whole "What have neutrinos done for you lately" concept from Conrads side troubles me a little. Has science (or perhaps grants) come to the point that science for the sake of knowledge is no longer enough? Must it lead to tangible benefits and inventions?

Edit to clarify: I know LHC and other experiments are pure basic research sake. It is just sad to see scientists feel the need to justify work that "only" yields knowledge.

Real-world applicability sometimes only gets token mention, but it does often get mentioned nonetheless. And based on personal experience, when you get stopped by random bozos in the parking lots who harangue you for wasting government money studying whatever it is you happen to be studying, you think about these sorts of things.

Fine article about neutrinos, but since we've got people doing materials engineering for fun, let's continue the side question. Lead is a poor choice for oversize construction since it's both very dense and very soft. Is there any material that could theoretically make a cylindrical frame light years long (effectively infinite) without collapsing under its own gravity? Titanium? Aerogel? Carbon gigatube?

Fine article about neutrinos, but since we've got people doing materials engineering for fun, let's continue the side question. Lead is a poor choice for oversize construction since it's both very dense and very soft. Is there any material that could theoretically make a cylindrical frame light years long (effectively infinite) without collapsing under its own gravity? Titanium? Aerogel? Carbon gigatube?

Hard question. You'd have to know Young's modulus for each material, and then prove that the derivative of the energy required to compress the material an infinitesimal amount:

dUt/dx ~ -(Y*pi*Rcyl^2/Lcyl) * x = Ft

is greater than the gravitational force trying to compress the cylinder.

You'll note that, the longer the cylinder, the smaller the tensile resistance to a compression, and of course the longer the cylinder, the more mass we have as well. It's not looking good.

The integral for the gravitational force from a cylinder is a beast. I leave it to the reader. Look to E&M for inspiration.

You mean apart from apart from better sanitation, medicine, education, irrigation, public health, roads, a freshwater system, baths and public order?

Really, this is asking "should pure science be pursued?" It's not like pure scientific research has ever delivered anything other than the stuff we take for granted every day like cars, microwave ovens, computers, washing machines, phones etc.

The fact that we won't see tangible, direct benefits from "pure" scientific research within the next 20 years shouldn't leave us to ask whether that research is worthwhile. This is the kind of research where you don't know until you get there what the value is, but provides ultimate returns many times greater than specific, focussed research.

Unfortunately it's all too hard to say that "this research produced those outcomes" with pure research, and so it's very easy for governments and grants providers to say "we're not seeing any value".

Neutrino masses don't really invalidate the standard model. The masses are just parameters in the model like the masses of all the other particles. It's just that in this case they were assumed to be zero. There is some subtlety about whether they would be Majorana or not, but the standard model stays basically the same.

Great article and now I have two bits of knowledge to take away. Scientists have more fun when the numbers don't agree, and I will spend far too long contemplating a 10,000 light-year long cylinder of lead.

I think asking about the utility of neutrino work isn't that terrible. Sure, 99.9999% of it is simply the accumulation of knowledge, but this stuff could have an amazing use in the future.

"And the toothpaste I mentioned at the beginning? Well, it turns out that certain isotopes located in odd little niches of the periodic table can get skipped for one reason or another; examples include lanthanum, tantalum, and, way down at the beginning, fluorine. These elements are made through a process known as neutrino spallation, or the neutrino process.

Spallation, which means “to flake,” occurs when a high-energy neutrino strikes a nucleus and knocks out a nucleon via the neutral current interaction; in the case of fluorine, a neutrino hits a neon-20 nucleus and flakes off a proton, leaving behind fluorine-19. Since neutrinos are shy and reluctant to interact with matter, neutrino spallation requires a high energy, high intensity neutrino wind illuminating a large quantity of enriched material. These conditions are achieved just before the shockwave rushes out of the center of a core-collapse supernova, when the dying core emits a brief, extremely intense pulse of neutrinos (the same pulse we detected from Supernova 1987A). As the neutrino pulse passes through the overlying star, it creates spallation elements along the way, including fluorine. Then the shockwave follows, blowing away the outer layers of the star into the interstellar medium, where the liberated fluorine can mix with gas and form into clouds, then protostellar cores, then stellar disks, then planets, then rocks, where some enterprising individual may pick them up and grind them down and mix them in with hydrated silica to form – your morning toothpaste."

Quote:

A neutrino with mass is the clearest indication that the Standard Model has to be either wrong or incomplete. In fact, a physicist in the audience pointed out that, had we known neutrinos had mass early enough, we might never have accepted the Standard Model in the first place.

But eventually we would have been forced to, as it is the effective model at least up to LHC energies.

And it isn't as if the Standard Model is complete on mass. Theoretical physicist and LHC coworker Matt Strassler of "Of Particular Significance" points out in one of his articles that while the standard Higgs* predicts much of SM model mass acquisition besides the neutrinos, it doesn't predicts its own mass acquisition!

"In order to really drive this point home, let me reintroduce two particles to you: the electron and the positron. You already know that the positron is the anti-partner of the electron… but for now, pretend you didn’t know that. The electron is left-chiral, while the positron is right-chiral. They’re two completely different particles.

How different are these particles? Well, the electron has electric charge -1, while the positron has electric charge +1. Further, the electron can couple to a neutrino through the W-boson, while the positron cannot.

Why does the W only talk to the (left-chiral) electron? That’s just the way the Standard Model is constructed; the left-chiral electron is charged under the weak force whereas the right-chiral positron is not. Note that at this stage, even the electron and the anti-positron are not the same particle! Even though they both have the same charge and chirality, the electron can talk to a W, whereas the anti-positron cannot.

For now let us assume that all of these particles are massless so that these chirality states can be identified with their helicity states. Further, at this stage, the electron has its own anti-particle (an “anti-electron”) which has right-chirality which couples to the W boson. The positron also has a different antiparticle, (an “anti-positron”) which has left-chirality but does not couple to the W boson.

We thus have a total of four particles (plus the four with opposite helicities):

Now here’s the magical step: masses cause different particles to “mix” with one another.*

This is very important; two completely different particles (the electron and the anti-positron) are swapping back and forth. What does this mean? The physical thing which is propagating through space is a mixture of the two particles. When you observe the particle at one point, it may be an electron, but if you observe it a moment later, the very same particle might manifest itself as an anti-positron! This should sound very familiar, it’s the exact same story as neutrino mixing (or, similarly, meson mixing).

Let us call this propagating particle is a “physical electron.” The mass-basis-electron can either be an electron or an anti-positron when you observe it; it is a quantum mixture of both. ...

Note that we can now say that the “physical electron” and the “physical positron” are antiparticles of one another. This is clear since the two particles which combine to make up a physical electron are the antiparticles of the two particles which combine to make up the physical positron."

To sum up so far (and I recommend the article and its cute particle graphics [that I omitted]!), there are several kinds of "antiparticles" around. Massive particles like neutrinos are mixtures of massless particles and antiparticles. The Standard Model differentiates between left and right chiral (massive) particles.

Hence *massive* fundamental particles can't be their own antiparticles. But:

"The kind of fermion [the kind of particles that makes up stuff; photons are massless bosons] mass that we discussed above is called a Dirac mass. This is a type of mass that connects two different particles (electron and anti-positron). It is also possible to have a mass that connects two of the same kind of particle, this is called a Majorana mass.

This type of mass is forbidden for particles that have any type of charge. For example, an electron and an anti-electron cannot mix because they have opposite electric charge, as we discussed above. There is, however, one type of matter particle in the Standard Model which does not carry any charge: the neutrino! (Neutrinos do carry weak charge, but this is “soaked up” by the Higgs vev.)"

As a side note, since neutrinos interact with the Higgs vev (vacuum expectation value) to loose its innateweak charge, I find it satisfying if it also interacts with the Higgs vev to gain its mass charge as the majority of the SM particles do.-------------------* This is where the Higgs particle comes in, as the "Higgs vev" gives particles mass by flipping them between the two massless particle states, see the article and its links.

Interesting article, thanks. While I have nothing to add to the topic, I would like to say this article's comment thread has been one of the most informative ones I've read in a long time. It reflects well the caliber of knowledge in this community.

I would like to say this article's comment thread has been one of the most informative ones I've read in a long time.

Vigorously seconded! I could barely follow much of it but it's quite interesting nonetheless.

I'd like to bring up the obvious layman's question here: since neutrinos do indeed have mass can they be a contributing factor to what we call dark matter?

Neutrinos probably account for slightly more mass than all baryonic matter (atoms and protons) put together. However, both combined are maybe 30% (at most) of all mass. 70% is still unaccounted for, and thus "dark".

Im just happy that we still have government funding to do research for the sake of knowledge. Granted it would be nicer if the general public felt that knowledge for the sake of knowledge is good, but these days I'll take what I can get.

I would like to say this article's comment thread has been one of the most informative ones I've read in a long time.

Vigorously seconded! I could barely follow much of it but it's quite interesting nonetheless.

I'd like to bring up the obvious layman's question here: since neutrinos do indeed have mass can they be a contributing factor to what we call dark matter?

I was pondering the same thing, but do neutrions stand still? Given the low mass pr particle, there would have to be a whole lot of them present.

I believe they always travel at the speed of light. You're right.. now I'm thinking there probably isn't a relationship unless a huge number of neutrinos are somehow orbiting the outskirts of galaxies. Oh well, dark matter remains mysterious for another day .

I believe they always travel at the speed of light. You're right.. now I'm thinking there probably isn't a relationship unless a huge number of neutrinos are somehow orbiting the outskirts of galaxies. Oh well, dark matter remains mysterious for another day .

They travel roughly at C, the speed of light in a VACUUM. Since photons interact with matter and neutrinos really don't, neutrinos can actually travel FASTER than the speed of light in a MEDIUM, like water. The blue glow given off by nuclear reactors is Cerenkov radiation emitted by neutrinos traveling faster than light. As a shorthand, Cerenkov radiation is to superluminal travel as shock waves are to supersonic travel.

I believe they always travel at the speed of light. You're right.. now I'm thinking there probably isn't a relationship unless a huge number of neutrinos are somehow orbiting the outskirts of galaxies. Oh well, dark matter remains mysterious for another day .

They travel roughly at C, the speed of light in a VACUUM. Since photons interact with matter and neutrinos really don't, neutrinos can actually travel FASTER than the speed of light in a MEDIUM, like water. The blue glow given off by nuclear reactors is Cerenkov radiation emitted by neutrinos traveling faster than light. As a shorthand, Cerenkov radiation is to superluminal travel as shock waves are to supersonic travel.

I'm not thinking this is entirely correct but neither am I a physicist. The blue light you see in a water reactor vessel is the result of high energy electrons, not necessarily neutrinos. Light travels at roughly .75C in water. These electrons are traveling at a value above .75C but not exceeding C. That's the shockwave in question that emits a blue light. Underground neutrino detectors that use photomultipliers to detect Cherenokov radiation actually detect the high energy electron, or muon, that results in the rare collision of a neutrino with one of the atoms in the water that is used as a detection medium.

I believe they always travel at the speed of light. You're right.. now I'm thinking there probably isn't a relationship unless a huge number of neutrinos are somehow orbiting the outskirts of galaxies. Oh well, dark matter remains mysterious for another day .

They travel roughly at C, the speed of light in a VACUUM. Since photons interact with matter and neutrinos really don't, neutrinos can actually travel FASTER than the speed of light in a MEDIUM, like water. The blue glow given off by nuclear reactors is Cerenkov radiation emitted by neutrinos traveling faster than light. As a shorthand, Cerenkov radiation is to superluminal travel as shock waves are to supersonic travel.

I'm not thinking this is entirely correct but neither am I a physicist. The blue light you see in a water reactor vessel is the result of high energy electrons, not necessarily neutrinos. Light travels at roughly .75C in water. These electrons are traveling at a value above .75C but not exceeding C. That's the shockwave in question that emits a blue light. Underground neutrino detectors that use photomultipliers to detect Cherenokov radiation actually detect the high energy electron, or muon, that results in the rare collision of a neutrino with one of the atoms in the water that is used as a detection medium.

Exactly, only charged particles emit Cerenkov radiation. Otherwise, electrons would be capable of shooting off gluons (analogously). If you ever have a physical result where neutrinos are directly interacting with photons, something has gone horribly wrong.

I'd like to bring up the obvious layman's question here: since neutrinos do indeed have mass can they be a contributing factor to what we call dark matter?

I read in one of the threads here that because neutrinos have low mass and travel at the speed of light, galaxies don't enough mass to trap them; that is neutrinos exceed the escape velocity for any galaxy. Therefore neutrinos can't account for the discrepancy in galaxy rotation curves as that would have to be caused by some sort of matter that has accumulated in/around the galaxy.